Scaling Soil Survey Data in the Sierra Foothills for Detailed Soil Resource Inventory, Mapping, and Interpretation.

The Sierra Foothill Region represents an important interface between the agricultural and urban areas of the Great Valley, and the wildlands of the Sierra Nevada. Although the rangelands of the Sierra Foothills have been mapped for most of California, the spatial detail and map unit complexity do not always adequately describe these highly variable soil landscapes. Over two-thirds of the state's water supply passes through the foothill regions, thus the relationships between soils and hydrology need to be documented at finer scales.  Considerable progress (mostly in academic circles) has been made in numerical approaches to soil mapping, however little has been directly integrated into current soil survey operations in California. The overall objective of this project was to evaluate digital soil mapping techniques (based on commonly available environmental data sources) to downscale soil survey information in the Sierra Foothill Region of California. Three benchmark catenas were established along a lithosequence across the foothill region, where intensive sampling was used to identify dominant soil-forming factors and processes that affect soil variability at the landscape scale. Indicies of terrain shape, geomorphic elements and microclimate gradients were extracted from an elevation model built from real-time kinematic (RTK) GPS measurements within each benchmark landscape. Canopy cover maps where derived from supervized classification (SMAP) of comnmonly available (NAIP) aerial imagery. Local (terrain-influenced) deviations from mean annual temperature and mean annual precipitation were evaluated by in-situ monitoring of volumetric water content and soil temperature. Multiple linear regression was used to establish and extrapolate soil-landscape and soil-microclimate relationships. A unique soil profile classification algorithm was used to make the final link between fine-scale maps of predicted soil properties and original map unit hierarchy. Geomorphic elements proved to be the most efficient down-scaling mechanism within each site, resulting in maps of expected soil component locations.